1. Field of the Invention
The present invention relates to a microminiature, monolithic, variable electrical device, and, more particularly, to such a device constituted of a basic "building block" comprising a deformable-mirror spatial light modulator ("SLM") functioning as a capacitor or switch.
2. Prior Art
An SLM is made up of an array of small mirrors or reflectors, each of which is capable of acting as a selective light-reflective pixel. Each pixel reflects incident light along a path which depends on the position or orientation of its mirror. Typically, each pixel mirror is movable (e.g. by deflection or deformation) between a normal, first position or orientation and one or more second positions or orientations. In only one position--either the normal position or one of the second positions--each pixel directs the incident light along a selected path to a primary light-receiving site, for example, into an optical system and from there onto a viewing surface or light-sensitive paper. In all other pixel mirror positions, incident light is not directed along the selected path to the primary site; rather, it is directed to either a secondary site or to a "light sink" which absorbs or eliminates the light which, therefore, does not reach the light-receiving site.
An array of pixels may be used to reflect incident light in a pattern to the primary site. A pixel array may take the form of a square or other othogonal matrix. In this event, the position of each pixel mirror, which position is individually controllable by associated addressing facilities, may be altered in a rasterized display to generate a video presentation. See commonly assigned U.S. Pat. Nos., 5,079,544; 5,061,049; 4,728,185 and 3,600,798. See also U.S. Pat. Nos. 4,356,730; 4,229,732; 3,896,338 and 3,886,310. A pixel array may also take other forms, for example, that of a rectangular matrix, the length of which is much greater than its width. In this latter event, the positions of the pixel mirrors, as determined by their associated addressing facilities, may be individually, selectively altered so that the reflected light prints characters in quasi-line-at-a-time fashion on light sensitive paper. See commonly assigned U.S. Pat. Nos. 5,101,236 and 5,041,851. In both events, and in other use environments, appropriate arrays and configurations of pixels/mirrors enable SLM's to modulate light in amplitude-dominant or phase-dominant modes.
There are at least four genera of SLM's: electro-optic, magneto-optic, liquid crystal and deflectable (or deformable) mirror. The latter genus, often referred to as a DMD--Deflectable (or Deformable) Mirror Device or Digital Micromirror Device--includes a micromechanical array of electronically addressable mirror elements. The mirror elements are reflectors each of which is individually movable (e.g., deflectable or deformable), as by rotation, deformation or piston-like, up-and-down movement into selective reflecting configurations. As noted above, each mirror constitutes a pixel which is capable of mechanical movement (deflection or deformation) in response to an electrical input. Light incident on each mirror may be selectively modulated in its direction and/or phase by reflection from each selectively moved or positioned mirror. To date, DMD SLM's have found use in optical correlation (e.g., in Van der Lugt matched filter correlators), spectrum analysis, optical crossbar switching, frequency excision, high definition displays (e.g. television), display and display projection, xerographic printing and neural networks.
There are several species of the genus "DMD SLM", including cantilever- and torsion- beam type elastomer type, and membrane type. A fourth species of DMD SLM which is structurally related to both beam types, but is operationally related to the elastomer and membrane types, is the so-called flexure-beam type. Addressing--that is, selectively moving--the pixels of DMD SLM's has been achieved by electron-beam input, optically or, as preferred today, by monolithic, thin film or hybrid integrated circuits, which include MOS, CMOS and functionally similar devices.
Specifically, it has been found convenient to produce integrated addressing circuits monolithically with the pixels using conventional MOS/CMOS processing techniques to form the addressing circuits in and on a substrate (typically silicon) with the pixels spaced above the substrate. The addressing circuits can be planarized and overlain by their respective pixels to limit light penetration thereinto, thereby reducing light diffraction from the addressing circuits and from the substrate. The addressing circuits may affect pixel positions in analog, bistable (binary) and tristable fashions.
Cantilever-beam and torsion-beam types of DMD SLM's each comprise a relatively thick (for rigidity and low compliance) mirror or reflective metal member typically integral with and supported at its edges by one or two relatively thin (for high compliance) cantilever beams (or springs) or torsion beams (or springs). Each mirror is structurally supported by its beams and separated from its associated addressing circuit and from an control or address electrode which is a part of or controlled by the addressing circuit, by a spacer or support post to which the beams are connected or attached.
Absent a deflecting force applied to each mirror or metal member, the mirror is maintained in its normal position by its beam(s). When the control or address electrode is energized by having a voltage from the addressing circuit applied thereto, the resulting electric field moves a portion of the mirror aligned with the electrode along the field lines. Such movement results from coulombic or electrostatic attraction of the portion of the mirror toward (or less typically repulsion away from) the electrode. Cantilever or torsional bending occurs preferentially at the thin beam(s). Such bending stores potential energy in the beam(s) associated with the deflected mirror. The stored potential energy, which tends to return the mirror to its normal position, is effective to achieve such return when the control or address electrode no longer attracts (or repels) it.
Once the addressing circuit and its control or address electrode are formed in or on the substrate, a planarizing organic photo-resist may be spun onto the substrate. A thin metal layer, such as aluminum, is the formed on the smooth surface of the photoresist, and the layer is patterned to form precursors of the mirrors and their associated beams. The thickness of the mirror precursor, but not the beam precursors, may be increased by selective deposition, masking, etching and related MOS/CMOS-like procedures. The photoresist is removed from under the mirror and beam precursors to form a well or air gap between each mirror, on one hand, and its address electrodes and the substrate, on the other hand.
During deflection, the attracted mirror portions move into and out of the wells. The direction taken by reflected, incident light depends on the position or orientation of each mirror and, hence, on the energization state of the associated control or address electrode. In this type of DMD SLM the thick mirrors are and remain relatively fiat, with their positions or orientations relative to the incident light and the light-receiving site being selectively altered to determine the path of reflected light.
One early type of DMD SLM--the elastomer type--includes a metallized, relatively thick elastomer layer. A later, related type of DMD SLM includes a relatively thin metallized polymer membrane stretched over a spacer grid or other support structure. The undeformed planar elastomer layer separates the metal layer thereon from underlying addressing facilities. The spacer grid effected an air gap or separation between grid-delineated segments of the normally undeformed and planar membrane and corresponding underlying addressing facilities. Each segment of the metal layer on the elastomer and the membrane constitutes a pixel. Engergization of a control or address electrode associated with each metal layer each metal layer segment electrostatically attracts (or repels) the metal segment to curvilinearly deform the associated, normally fiat, related elastomer or membrane segment out of its normal, undeformed, planar configuration and toward (or away from) the electrode, whereupon the curvilinearly deformed metal segment acts as a miniature spherical, parabolic or other curved mirror.
Deformation of the elastomer and membrane stores potential energy therein. Deenergization of the control or address electrode permits the stored potential energy in the elastomer and membrane segment to return it to its normal fiat configuration. Incident light reflected by each miniature mirror may be concentrated into a relatively narrow cone that is rotationally symmetric. Each pixel could, therefore, be associated with a Schlieren stop, comprising a single, central obscuration having a position and size to block the light reflected by the fiat or unmodulated pixel mirrors. Modulated, curved or deformed pixel mirrors direct a circular patch of light onto the plane of the stop; the patch is centered on, but is now larger than, the stop's central obscuration and, therefore, traverse a selected direction and reaches a selected site.
As with DMD SLM's of the beam type, DMD SLM's of the membrane type have also recently been produced by forming a hybrid integrated assembly comprising an array of relatively thick, low compliance, separated, fiat pixel mirrors each supported by relatively thin high compliance members. The members may, as in the past, be metallized segments of a polymer membrane or separate metallized polymer membranes. More typically, the members are segments of a compliant metal membrane or thin, stretchable and highly compliant or projections connected to or integral with their mirrors. The metal projections (or metal membrane, as the case may be) space the mirrors a first distance above a silicon or other substrate having formed therein and thereon addressing circuits. Underlying addressing circuits are separated by air gaps from their associated pixel mirrors when the latter reside in their normal positions. When an addressing circuit is appropriately energized, its pixel mirror is displaced or deflected toward the substrate by electrostatic or coulombic attraction. If the mirrors and the metal membrane or the metal projections are of similar thinness, the displaced mirror curvelinearly deforms. If the mirrors are substantially thicker than the surrounding metal membrane or the metal projections, each displaced mirror remains essentially fiat while the metal projections (or the metal membrane) immediately stretch and deform to permit the mirrors to deflect up-and-down in piston-like fashion. The resultant displacement pattern--of the curvelinearly or transversely displaced mirrors--produces a corresponding amplitude or phase modulation pattern for reflected light.
A DMD SLM of the flexure-beam type includes a relatively thick fiat mirror supported by a plurality of relatively thin cantilever-torsion beams. In an exemplary flexure-beam type of DMD SLM, the mirror is a rectangle or a square and each beam extends partially along a respective side of the mirror from a spacer or support post to a comer of the beam. In this type of SLM the beams extend parallel to the mirror's sides, while in the cantilever- and torsion-beam SLM's, the beams typically extend generally perpendicularly or acutely away from the sides of the mirrors.
When a mirror of a flexure-beam device is attracted by its control or address electrode, the beams undergo primary cantilever bending and secondary torsional bending to effect piston-like movement or deflection of the fiat mirror with very slight turning of the flat mirror about an axis parallel to the direction of piston-like deflection and perpendicular to the mirror.
Further general information on SLM's may be obtained from a paper entitled "Deformable-Mirror Spatial Light Modulators," by Larry J. Hornbeck, presented at the SPIE Critical Review Series, Spatial Light Modulators and Applications III, in San Diego, Calif. on Aug. 7-8, 1989 and published in Volume 1150, No. 6, pages 86-102 of the related proceedings.
All DMD SLM's comprise an array of individually movable (deflectable or deformable) mirrors, pixels or light-reflecting surfaces. As discussed in commonly assigned U.S. Pat. No. 5,061,049, DMD's have been recognized as also comprising, in effect, air gap capacitors. Apparently, however, the capacitive nature of DMD SLM's has been relied on primarily for analysis of the operation of the DMD's. That is, while the optical characteristics of DMD SLM's have, and continue to be exploited, little work has been done which capitalizes on the inherent electrical or non-light-reflecting nature of these devices.
One object of the present invention is the provision of a microminiature, monolithic, variable electrical device, such as a capacitor or switch, comprising a DMD, qua or switch, and of various apparatus comprising or including such a device. Apparatus utilizing variable DMD devices, such as capacitors and switches, includes transmission lines (such as variable impedance microstrip lines); variable impedance matching, transforming and filter networks; variable-impedance or frequency-agile antennae (including patch, spiral and slot) which are tunable as to radiation pattern, frequency and wavelength; variable-impedance or frequency-agile couplers (including symmetric, asymmetric and rat race); variable FIN lines associated with waveguides; waveguides per se, switches for optical waveguides and electrical transmission lines; circuit operational controllers, for example, to tune compensate or control high frequency oscillators; and true time-delay networks for phased array antennas. Because of the operating mode of DMD's, various apparatus in which the DMD-derived devices of the present invention are included, may be digitally or selectively variable or tunable.